Loud speaker device



Aug. 18, 1931; 1. LUDLOW LOUD SPEAKER DEVICE Filed Jan. 14, 1926 INVENTOR By Attorneys,

35 natural.

Patented Aug. 18, 1931 UNITED STATES PATENT OFFICE ISRAEL LUD'LOW, OF NEW YORK, N. Y., ASSIGNOR OF ONE-HALF TO ALBERT E. LAMB.

OF NEW YORK, N. Y.

LOUD SPEAKER DEVICE Application filed January 14, 1926. Serial No. 81,161.

This invention relates to a loud speaker device for use in reproducing sound.

The object of the invention is to provide an improved loud speaker capable of reproducing audible sounds over a Wide range as a source of sound waves if caused to vibrate in free air. It is well understood that .unless a large diaphragm is provided wit-h means for radial reinforcement, it will not act'to set up sound waves according to audio frequency vibrations that may be accurately applied to the center of the diaphragm by an electromagnetic loud speaker unit. The movements of the diaphragm, instead of following the vibrations of the actuating 39 unit, will be broken up into various irregular wave motions within itself. The audible response from such a diaphragm for a given energy input willbe relatively small and the quality of reproduction far from While various methods have been .employed to impart to a diaphragm adequate radial rigidity to prevent the lrregullar response to impressed vibrations, efforts in this direction have only been partially 7 successful because of the fact that great radial rigidity can only be obtained by constructions which have considerable mass.

The cone type of diaphragm possesses, per haps, a maximum radial rigidity considering the area of such reproducing elements,

.but even in the case of the conical dia- .phragm extraneous vibrations are set upwithin,and propagated as sound waves by such reproducers.

' In any type of loud speaker not employphragms.

ing a horn it is necessary to use a diaphragm of relatively large size in order to engage a sufiicient body of air to effectively propagate sound waves. A phonographdiaphragnnfor example, if vibrated in free air is capable ofemitting only comparatively limited sou'ndyenergy regardless of the intensity of vibratory force applied to it It being apparent that a diaphragm of comparatively large size is necessary for other than the horn type of. loud speaker and that large diaphragms ordinarily possess certain inherent defects resulting in distorted responses to applied vibratory energy, the improvement according to the present invention is important because it per- 'mits the use of a large diaphragm of comparatively small mass which, at the same time, is not subject to the distorting tendencies ordinarily present in such dia- I have found that a confined body of air of suitable dimensions with respect to the size and form of thediaphragm can be utilized to support the diaphragm; 111 such manner as to take the place of the usual devices for radial reinforcement, or, if such devices are employed, to supplement them in such manner as to render the response of the diaphragm more accurate, with:

a consequent improvement in the quality of sound reproduced. In other words, I employ a diaphragm which, in itself, possesses certain structural weaknesses, and

compensate for this deficiency by placing explanation of the operation of my invention is purely theoretical. Actual tests demonstrate that some acoustic principle is availed of in my invention to improve the reproduction from a large diaphragm. It may be that further study of the behavior of my loud speaker willafi'ord a better explanation for its novel performance.

The invention will now be described in detail with reference to the accompanying drawings, in which Figure 1 is a perspective view of a loud speaker according to my invention.

Fig. 2 is a crosssection of the device shown in Fig. 1 with a slightly modified'diaphragm suspension.

Fig. 3 is a. cross-section of a modified form of loud speaker. T

Fig. 4 is a cross-section of a further modification of the loud speaker according to my invention.

Fig. 5 is a vertical cross-section of a loud speaker similar to the device shown in Fig. 4 but having double oppositely-actuated diaphragms instead of only a single vibrating member.

Fig. 6 is a detailed, enlarged cross-section of the special actuating mechanism for the double diaphragm device shown in Fig. 5.

' According to the preferred form of my invention the loud speaker comprises a large, flat diaphragm 1 flexibly supported at its edges by means of resilient tie-pieces 2 which, in turn, are fastened toward the edge of a cupped reflector 3. Any type of electromagnetic diaphragm actuating unit 5 may be employed, the unit being positioned, as shown, within the cupped reflector and centrally thereof and connected to the diaphragm by suitable means such as are ordinarily employed for this purpose. The loud speaker unit 5 is mounted on brackets .6, which may be screwed or otherwise fastened securely to the reflector 3. The flexible tiemembers 2 supporting the diaphragm are preferably connected tothe reflector through an adjusting means comprising a screwthreaded element 7 by the adjustment of which the hole 8 in the center of the diaphragm, throughwhich the diaphragm actuating pin 9 of the loud speaker unit must pass, may be brought into accurate alignment with such pin; Y

It will be observed that the diaphragm does not completely close the cupped reflector but is of a sufficiently reduced diameter to provide an air gap between the outer edge of the diaphragm and the edge of the cupped reflector. With a diaphragm approximately ten inches in diameter a gap between the diaphragm and the nearest surface of the cupped reflector of approximately onequarter of an inch has been found to be the best. The size of this gap will vary with different sized diaphragms. Reference to the above described gap between the diaphragm and reflector will hereinafter be simplified by designating this separation as the gap 10, the numeral appearing in Fig. 1.

The reflector is preferably provided with guard members 1]., which maybe made in any convenient form and preferably pass completely across the face of the diaphragm,

being rigidly attached to the edges of the reflector. These members may bear any desired ornamentation, thereby improving the appearance of the loud speaker and at the same time afford substantial protection to the diaphragm against possible derangement through accidental contact therewith by an outside object. The loud speaker is preferably mounted, as shown, upon a base 12 which has a suflicient bearing to prevent the instrument from being accidentally overturned.

I have found that in constructing a loud speaker in accordance with the design shown in Fig. 1 the annular gap 10 pro vided between the edge of the diaphragm and the sound-reflector may be covered with a light fabric which may be cemented around the edge of the diaphragm and fastened to the walls of the'reflector in any because of the possibility of the propagation and diffusion of "sound waves from the rear or internal surface of the diaphragm 1 outward through the annular gap 10, the light fabric closure '15 ofthe gap 10 having been found not to materially interfere with the transmission of' sound from the reflector chamber outward through the annular gap surroundingthe diaphragm.

It is to be borne in mind that the relatively high'efliciency of the loud speaker shown in Figs. 1 and 2 is attributable to two factors. One is the fact that the sound from the rear of the diaphragm, which is ordinarily propagated in' a direction op osite to that in which the sound is wanted, 18, by means of the rigid reflector 3 and the annular gap 10 surrounding the diaphragm, redirected outward into space in substan tially the direction of the sound normally produced at the front of the diaphragm. The other factor making for increased efficiency with regard to the volume of sound produced for a given-energy applied to'actuate the diaphragm is the fact that-while a comparatively rigid diaphragm is employed, its edges are mounted in such mannor as to permit the diaphragm to freely 'vibrate more nearly after the fashion of-=a piston than a diaphragm, the edges of which latter device are ordinarilysomewhat rigidly'clainped in a mounting. Also, as has hereinbefore been pointed out, the use of a larger diaphragm than could ordinarily be employed in a speaker of any particular design is permitted by virtue of what may be termed the acoustic support of the dia phragm which it receives from the reflector in front of which the diaphragm is mounted. As is well understood, up to a point not yet reached, the larger the diaphragm that can be successfully caused to follow an impressed actuating vibration the greater will be the volume of sound produced.

In Fig. 3 is shown a modification of my invention in which a conical diaphragm 20 is mounted coaxially with, and at a small distance from, a conical reflector 21. Be.- cause of the inherent radial rigidity of a conical diaphragm this element may be constructed of paper or parchment in the manner commonly employed with present cone speakers. The free edges of the cone 20 are preferably suspended at various points by flexible connections 22 to the surrounding reflector.

The conical diaphragm may be actuated in any suitable manner, a convenient placing of the electromagnetic actuating unit being shown, the unit 23 being fastened centrally and at the apex of the conical reflector 21, a flattened portion being provided toward the apex of the reflector against which the actuating unit 23 is fastened.

The conical reflector 21 may be formed of sufliciently heavy gauge sheet-metal to render it free from undue response to the sound energy thrown against it by the diaphragm, or it may be constructed of wood or molded from some fairly dense plastic material such as bakelite.- As in the modification previ ously described, the diaphragm may be attached to the supporting reflector by means of an annular section of light fabric, which also provides a dust-proof closure for the space between the rear of the diaphragm and the conical reflector.

A conical loud speaker of the type shown in Fig. 3 has, perhaps, a maximum output of sound for a given energy input from the actuating unit because of the placing of the inner surface of the conical diaphragm in close proximity to the conical surface of the reflector 21, a comparatively slight movement of the diaphragm enforcing a far more intense expulsion of air in the form of a sound wave from the annular opening between the edge of the diaphragm and the surrounding reflector, than is .ob-

tained where considerable space is provided between the surface of the diaphragm and the reflector in front of which it is'mounted. The proper spacing between the conical diaphragm and the reflector within which 5- it is mounted will be governed by severali variable factors such as the thickness of 'ener from the rear or enclosed surface iof 7o the conical diaphragm, a damping effect will be observed which will prevent efficient response of the diaphragm to impressed vibrations. On the other hand, if the spacing is made too great, the efficiency of the is loud speaker will be somewhat impaired.

According to the preferred embodiment of the application of my invention to the cone type of loud speaker, the conical diaphragm is made' somewhat more pointed o than the cone of the reflector in which it is mounted, thereby providing a tapering annular space or chamber between these two elements, the cross-section of which increases toward the open or large end of there-"s5 flector, thereby providing for the normal expansion of the sound wave as it travels from the apex of the cone outward toward the base, thus following the usual practice in the construction of chambers for sound' eo producing or reproducing instruments such, for example, as many of the wind instruments in which the horn is flared toward the open end. While it is true that the space between two concentrically disposed c5 cones does increase in cross-sectional area toward the base of the cone, this increase is in substantially direct proportion to the length of the path followed by a sound wave in passing from the apex of the dia-"ioo phragm to the base. This increase in crosssection is not sufliciently rapid to meet the requirement of the expanding sound wave and consequently is supplemented by providing 'for an increased expansion througlnio5 the difference in taper of the diaphragm and reflecting cone. This combination affords a path for the sound energy corresponding in cross-sectional increase to that alforded by a so-called morning glory horn, whereas in the case where a diaphragm and reflector of the same taper are used together,

the increase in cross-sectional area of the space between the two as the wave progresses from the apex toward the base of the cone in; 7 corresponds to the uniform increase in crosssectional area of a perfectly straight tapered horn, which type of horn is known not to be as effective for the propagation of sound waves as the former type described. 1

A third form of my loud speaker, employing a substantially spherically dished diaphragm, is illustrated in Fig. 4, theconstruction following closely that shown in Fig. '3, the only material difference being the shape of the diaphragm and the reflector with which it cooperates. According to the present modification, the diaphragm is dished to form a bowl. This bowl is suspended in front of a reflector the internal surface of which corresponds to the'shape of the diaphragm, the two elements being spaced, as shown, to provide an annular conduit 25 through which the sound propagated by the surface of the diaphragm adjacent to the reflector is led outwards and directed along lines substantially parallel to the course of sound produced from. the opposite surface of the conical diaphragm.

The loud speaker actuating unit 26 may be provided with a flange 27 through which screws 28 may pass and be made fast to the body of the bowl-shaped reflector 29. The screws 28 pass through holes 30 in the flange 27, which holes are somewhat larger than the threaded portion of the screw and thus permit of a movement of the loud speaker unit within limits, whereby the actuating rod 31, connecting the electromagnetic driving'mechanism of the unit with the diaphragm, may be adjusted so as to bring it into exact alignment with the center of the diaphragm 32 where it is fastened in any suitable manner.

Another modification of my invention is illustrated in Fig. 5, the device there shown comprising a central horizontally disposed reflector which consists of two cones 40 and 41 placed base to base. These two cones are formed preferably of some rigid material of 'such character as to afford effective surfaces for sound deflection. Above and below the two reflector cones 40 and 41, and in coaxial alignment therewith, are mounted conical diaphragms 42 and 43, respectively. Thus far the arrangement corresponds to a simple duplication of the loud speaker shown in Fig. 3 except for the fact that the two cones of the present modification are mounted horizontally instead of vertically. This arrangement has the advantage of distributing sound with equal intensity in every direction in a horizontal plane, and has, to a certain extent, the effect of concentrating the sound energy in a horizontal plane. The diaphragms of the device shown in Fig. 5 are actuated by two separate loud speaker units the arrangement of which is such as to at all times move the two diaphragms oppositely, that is, when the upper diaphragm is moving upwards the lower diaphragm will move downwards, and, conversely, when the upper diaphragm moves downwards the lower diaphragm will move upwards. This has the effect of propagating sound waves from the two diaphragms cooperating with their respective reflectors,

which waves have the same sign, both diaphragms simultaneously producing waves of compression and rarefaction, whereas loud speakers using double diaphragms ordinarily move the diaphragms in the same direction, so that when one is producing a wave of compression the other is producing a wave of rarefaction, with the result that these two waves tend somewhat to produce interference.

The two diaphragms 42 and 43, according to the embodiment illustrated in Fig."

5, may be mounted in any suitable manner 'upon two annular frames 37 and 38, respectively, which frames are spaced apart and positioned with respect to the double conical reflector 40 by means of distance pieces 39, upon which pieces the rigid reflector 4041 is also mounted, as shown. A base 36 is provided with supporting arms which are fastened to the distance pieces 39 and thereby afford a support for the entire" instrument. The two loud speaker units 34, 34 within the reflector 40 may be rigidly attached to the inner surfaces of the reflector by means of supporting plates 33, as shown, attached to the upper and lower cones of the reflector.

In Fig. 6 there is shown an arrangement by which the two diaphragms 42 and 43 may be oppositely actuated by means of a single actuating unit. An ordinary electromagnetic loud speaker mechanism may have its actuating rod 45 commonly connected to two bell-cranks 46 and 47, the free ends of which, as shown, are connected respectively to diaphragms 42 and 43 by means of small links 48. The electro-magnetic unit may merely actuate a simple armature to which the connecting rod 45 is attached (see Fig. 4), as is commonly the case in units designed primarily to actuate the cone type of' loud speaker, or there may be a diaphragm 49 within the loud speaker unit which isv actuated electromagnetically according to the usual practice in units designed to be employed with the horn type of loud speaker, and the bell-crank mechanism, above described, connected by a suitable link to such diaphragm, as indicated in Fig. 6.

The type of unit in which no diaphragm is employed is preferable both because of its" 1. A sound reproducing device including a diaphragm, actuating means applying energy to said diaphragm substantially at its center, one surface of said diaphragm being adapted to act directly on free and unconfined air, and a substantially fixed and nonvibrating sound reflector coaxial with and spaced apart from said diaphragm and forming in conjunction therewith a chamber which at least partially encloses a body of air, the said reflector being marginally separated from the diaphragm so as to provide an annular passage between the edge of the diaphragm and said reflector, through which passage sound energy set up between said diaphragm and reflector may freely pass.

2. A sound reproducing device according to claim 1, further characterized in that the shape of the diaphragm and the reflector cooperating therewith is such as to provide a greater separation between such two elements toward their peripheries than toward their centers, the actuating means applying vibrations substantially perpendicularly to the basal plane of the diaphragm.

3. A sound reproducing device according to claim 1, further characterized in that the diaghragm and reflector are interfitting conical elements, the actuating means applying vibrations substantially perpendicularly to the basal plane of the diaphragm.

1. A sound reproducing device according to claim 1, further characterized in that the diaphragm and reflector are interfitting conical elements, the apex angle of the inner conical element being smaller than the apex angle of the outer conical element whereby there is provided between them a tapered annular air chamber, the separation of the walls of which is greater toward the periphery of said elements, than toward the center, the actuating means applying vibrations substantially perpendicularly to the basal plane of the diaphragm.

5. A sound reproducing device according to claim 1, further characterized in that the said diaphragm is made of sheet-metal.

6. A sound reproducing device according to claim 1, further characterized in that the said diaphragm is relatively stiff and is supported at its edges by yielding means which permit of comparatively free bodily displacement of the diaphragm in response to vibrations applied perpendicularly to the plane of the diaphragm.

7. A sound reproducing device including two diaphragms of sufliciently large size to effectively set up sound waves by direct action upon free air, and a substantially nonvibrating reflector between said diaphragms.

In witness whereof, I have hereunto signed my name.

ISRAEL LUDLOW. 

